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TAMU MATH 304 - Lect3-11web

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MATH 304Linear AlgebraLecture 34:Review for Test 2.Topics for Test 2Linear transformations (Leon 4.1–4.3)• Matrix transformations• Matrix of a linear mapping• Similar matricesOrthogonality (Leon 5.1–5.6)• Inner products and norms• Orthogonal complement• Least squares problems• The Gram-Schmidt orthogonalization processEigenvalues and eigenvectors (Leon 6.1, 6.3)• Eigenvalues, eigenvectors, eigenspaces• Characteristic polynomial• DiagonalizationSample problems for Test 2Problem 1 (20 pts.) Let M2,2(R) denote the vector spaceof 2 × 2 matrices with real entries. Consider a linear operatorL : M2,2(R) → M2,2(R) given byLx yz w=x yz w1 23 4.Find the matrix of the operator L with respect to the basisE1=1 00 0, E2=0 10 0, E3=0 01 0, E4=0 00 1.Problem 2 (30 pts.) Let V be a subspace of R4spannedby the vectors x1= (1, 1, 1, 1) and x2= (1, 0, 3, 0).(i) Find an orthonormal basis for V .(ii) Find an orthonormal basis for the orthogonal complementV⊥.Problem 3 (30 pts.) Let A =1 2 01 1 10 2 1.(i) Find all eigenvalues of the matrix A.(ii) For each eigenva l ue of A, find an associated eigenvector.(iii) Is the matrix A diagonalizable? Explain.(iv) Find all eigenvalues of the matrix A2.Bonus Problem 4 (20 pts.) Find a linear polynomial whichis the best least squares fit to the following data:x−2 −1 0 1 2f (x) −3 −2 1 2 5Bonus Problem 5 (20 pts.) Let L : V → W be a linearmapping of a finite-dimensional vector space V to a vectorspace W . Show thatdim Range(L) + dim ker(L) = dim V .Problem 1 (20 pts.) Let M2,2(R) denote the vector spaceof 2× 2 matrices with real entries. Consider a linear operatorL : M2,2(R) → M2,2(R) given byLx yz w=x yz w1 23 4.Find the matrix of the operator L with respect to the basisE1=1 00 0, E2=0 10 0, E3=0 01 0, E4=0 00 1.Let MLdenote the desired matrix.By definition, MLis a 4×4 matrix whose columns arecoordinates of the matrices L(E1), L(E2), L(E3), L(E4)with respect to the basis E1, E2, E3, E4.L(E1) =1 00 01 23 4=1 20 0= 1E1+ 2 E2+ 0 E3+ 0 E4,L(E2) =0 10 01 23 4=3 40 0= 3E1+ 4 E2+ 0 E3+ 0 E4,L(E3) =0 01 01 23 4=0 01 2= 0E1+0E2+1E3+2E4,L(E4) =0 00 11 23 4=0 03 4= 0E1+0E2+3E3+4E4.It follows thatML=1 3 0 02 4 0 00 0 1 30 0 2 4.Thus the relationx1y1z1w1=x yz w1 23 4is equivalent to the relationx1y1z1w1=1 3 0 02 4 0 00 0 1 30 0 2 4xyzw.Problem 2 (30 pts.) Let V be a subspace of R4spannedby the vectors x1= (1, 1, 1, 1) and x2= (1, 0, 3, 0).(i) Find an orthonormal basis for V .First we apply the Gram-Schmidt orthogonalization process tovectors x1, x2and obtain an orthogonal basis v1, v2for thesubspace V :v1= x1= (1, 1, 1, 1),v2= x2−x2· v1v1· v1v1= (1, 0, 3, 0)−44(1, 1, 1, 1) = (0, −1, 2, −1).Then we normalize vectors v1, v2to obtain an orthonormalbasis w1, w2for V :kv1k = 2 =⇒ w1=v1kv1k=12(1, 1, 1, 1)kv2k =√6 =⇒ w2=v2kv2k=1√6(0, −1, 2, −1)Problem 2 (30 pts.) Let V be a subspace of R4spannedby the vectors x1= (1, 1, 1, 1) and x2= (1, 0, 3, 0).(ii) Find an orthonormal basis for the orthogonal complementV⊥.Since the subspace V is spanned by vectors (1, 1, 1 , 1) and(1, 0, 3, 0), it is the row space of the matrixA =1 1 1 11 0 3 0.Then the orthogonal complement V⊥is the nullspace of A.To find the nullspace, we convert the matrix A to reduced rowechelon form:1 1 1 11 0 3 0→1 0 3 01 1 1 1→1 0 3 001 −2 1.Hence a vector (x1, x2, x3, x4) ∈ R4belongs to V⊥if and onlyif1 0 3 00 1 −2 1x1x2x3x4=00⇐⇒x1+ 3x3= 0x2− 2x3+ x4= 0⇐⇒x1= −3x3x2= 2x3− x4The general solution of the system is (x1, x2, x3, x4) == (−3t, 2t − s, t, s) = t(−3, 2, 1, 0) + s(0, −1, 0, 1 ), wheret, s ∈ R.It follows that V⊥is spanned by vectors x3= (0, −1, 0, 1)and x4= (−3, 2, 1, 0).The vectors x3= (0, −1, 0, 1) and x4= (−3, 2, 1, 0) form abasis for the subspace V⊥.It remains to orthogonalize and normalize this basis:v3= x3= (0, −1, 0, 1),v4= x4−x4· v3v3· v3v3= (−3, 2, 1, 0) −−22(0, −1, 0, 1)= (−3, 1, 1, 1),kv3k =√2 =⇒ w3=v3kv3k=1√2(0, −1, 0, 1),kv4k =√12 = 2√3 =⇒ w4=v4kv4k=12√3(−3, 1, 1, 1).Thus the vectors w3=1√2(0, −1, 0, 1) andw4=12√3(−3, 1, 1, 1) form an orthonormal basis for V⊥.Problem 2 (30 pts.) Let V be a subspace of R4spannedby the vectors x1= (1, 1, 1, 1) and x2= (1, 0, 3, 0).(i) Find an orthonormal basis for V .(ii) Find an orthonormal basis for the orthogonal complementV⊥.Alternative solution: First we extend the set x1, x2to a basisx1, x2, x3, x4for R4. Then we orthogonalize and normalizethe latter. This yields an orthonormal basis w1, w2, w3, w4for R4.By construction, w1, w2is an orthonormal basis for V .It follows that w3, w4is an orthonormal basis for V⊥.The set x1= (1, 1, 1, 1), x2= (1, 0, 3, 0) can be extended toa basis for R4by adding two vectors from the standard basis.For example, we can add vectors e3= (0, 0, 1, 0) ande4= (0, 0, 0, 1). To show that x1, x2, e3, e4is indeed a basisfor R4, we check that the matrix whose rows are these vectorsis nonsingular:1 1 1 11 0 3 00 0 1 00 0 0 1= −1 3 00 1 00 0 1= −1 6= 0.To orthogonalize the ba sis x1, x2, e3, e4, we apply theGram-Schmidt process:v1= x1= (1, 1, 1, 1),v2= x2−x2· v1v1· v1v1= (1, 0, 3, 0)−44(1, 1, 1, 1) = (0, −1, 2, −1),v3= e3−e3· v1v1· v1v1−e3· v2v2· v2v2= (0, 0, 1, 0) −14(1, 1, 1, 1)−−26(0, −1, 2, −1) =−14,112,112,112=112(−3, 1, 1, 1),v4= e4−e4· v1v1· v1v1−e4· v2v2· v2v2−e4· v3v3· v3v3= (0, 0, 0, 1)−−14(1, 1, 1, 1) −−16(0, −1, 2, −1) −1/121/12·112(−3, 1, 1, 1) ==0, −12, 0,12=12(0, −1, 0, 1).It remains to normalize vectors v1= (1, 1, 1, 1),v2= (0, −1, 2, −1), v3=112(−3, 1, 1, 1), v4=12(0, −1, 0, 1):kv1k = 2 =⇒ w1=v1kv1k=12(1, 1, 1, 1)kv2k =√6 =⇒ w2=v2kv2k=1√6(0, −1, 2, −1)kv3k =1√12=12√3=⇒ w3=v3kv3k=12√3(−3, 1, 1, 1)kv4k =1√2=⇒ w4=v4kv4k=1√2(0, −1, 0, 1)Thus w1, w2is an orthonormal basis for V while w3, w4is anorthonormal basis for V⊥.Problem 3 (30 pts.) Let A =1 2 01 1 10 2 1.(i) Find all eigenvalues of the matrix A.The eigenvalues of A


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TAMU MATH 304 - Lect3-11web

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